ValidationService provides better support for customization

Description

The following API is for union, add, remove Validator in Validators list of each element.

Instead of using raw string manipulation like:

 
  return string.Concat(validators, Settings.Current.SelectorValueSeparator, pickerValidator);
  

• introduce ValidationService.CreateValidatorFromRawString()
• introduce ValidationService.CreateValidatorListFromRawString()

Remove static functions in FormsExtensions.Validation and put more functions in the ValidationService.